1. Field of Invention
The present invention relates in general to an apparatus and method for laying optical fiber unit. More specifically, the invention relates to sending an optical fiber unit by pressure.
2. Description of Related Art
Japanese Patent Unexamined Publication No. Sho-59-104607 discloses a conventional method for laying an optical fiber unit into a previously laid duct. FIG. 2 (PRIOR ART) illustrates that method. An optical fiber unit 23 is fed through an optical fiber unit inlet-side seal portion 26 of a pressure-sending device 21 and from there sent by a driving device 25 toward a duct line 22. The duct line 22 is connected with an air tight seal to the pressure-sending device 21. Pressure fluid fed from a pressure fluid feed inlet 24 to the pressure-sending device is fed into the duct line 22. It is convenient to use air as the pressure fluid. Accordingly, the optical fiber unit 23 advanced into the duct line 22 is moved by the flow of the pressure fluid.
In such a conventional pressure-sending method, the seal portion 26 is provided on the optical fiber unit inlet-side of the pressure-sending device so as to prevent the pressure fluid from flowing backward. This results in the pressure fluid flowing toward the advancing direction of the optical fiber unit only.
One embodiment of the optical fiber unit to be used with the method of the present application is disclosed in the above-mentioned Publication and is an optical fiber unit with a foam layer. When a foam layer is used as an outer-layer coat, it has the advantageous characteristics of both being light-weight and having an extremely uneven surface. Such an uneven surface provides a wide surface area so that a large force of pressure can be used to advance the optical fiber unit through the duct.
However, it has been found that the prior art embodiment described above has the following problems.
(1) Although the conventional optical fiber unit having a foamed-body surface is advantageously light-weight and has an uneven surface, it is desirable, at the seal portion, to have an outer layer having little fluctuation in its outer diameter on the optical fiber unit inlet side, to maintain air-tightness in the pressure-sending device. It is difficult to maintain a consistent outer diameter at high accuracy on the optical fiber when the foamed-body surface is used. Thus, two opposing characteristics are required for the outer layer of the optical fiber unit: uneven surface and consistent outer diameter.
(2) When the method described above is used for the wiring in a building or the like, the weight alone of the optical fiber unit causes resistance. Also, since the wiring in a building has many curves, the optical fiber unit must sometimes be pulled up by traction through a duct line to an upper floor with the pressure-sending device attached. In this case, as shown in a longitudinal section of FIG. 3(A) and a cross section of FIG. 3(B), a problem arises in that the force F for fastening an optical fiber unit 31 against a wall surface in the inside of curved portions of a duct line 32 increases friction force between the optical fiber unit 31 and the inner wall of the duct line 32, so that the optical fiber unit 31 cannot be pulled up.